1. Field of the Invention
The present invention relates to a semiconductor light-emitting device used for recording or reproduction of information on an optical disk which has self-pulsation characteristics for noise reduction, and to a method for producing such a device.
2. Description of the Related Art
As the storage capacity of optical disks has increased, there has arisen a need for a light source for optical disks having a narrower light-collecting diameter as compared to conventional light sources for optical disks so as to emit light having a wavelength of about 400 nm for recording information on an optical disk with high density. In an optical disk system, use of an inexpensive plastic material for a lens, a disk, etc., is considered for the purpose of reducing production costs of the optical system. However, a light absorption edge of such a plastic material is at a wavelength of up to about 390 nm. Accordingly, there is a necessity to study materials applicable to the light source for optical disks in order to achieve a shorter wavelength, and thus optical systems using such plastic material cannot be mass-produced. As a light source emitting light having such a short wavelength, conventionally, a semiconductor laser has been used. A representative material used for a semiconductor laser for emitting light having a wavelength of about 400 nm is a gallium nitride compound semiconductor.
A nitride semiconductor laser used in an optical disk system or the like has a structure which has self-pulsation characteristics in order to reduce the noise in optical feedback (hereinafter, also referred to as the xe2x80x9cexternal optical feedback noisexe2x80x9d) from a reflective point on an optical disk, etc. In order to realize such a nitride semiconductor laser involving self-pulsation, a layer having saturable absorption characteristics (hereinafter, referred to as a xe2x80x9csaturable absorbing layerxe2x80x9d) in which saturation of light absorption occurs is provided in a p-type cladding layer, etc., included in the nitride semiconductor laser.
FIG. 7 is a cross-sectional view of a representative structure of a low-noise semiconductor laser for optical disks as disclosed in Japanese Laid-Open Patent Publication No. 9-191160. This publication discloses a low-noise semiconductor laser capable of obtaining self-pulsation by using InGaN, which is a nitride semiconductor, and a mixed crystal of InN (indium nitride) and GaN (gallium nitride) for a saturable absorbing layer. As illustrated in FIG. 7, this low-noise semiconductor laser includes an n-type SiC substrate 70 on which an n-type AlN layer 71, an n-type AlGaN cladding layer 72, an n-type GaN light-guiding layer 73, an InGaN quantum well active layer 74, a p-type GaN light-guiding layer 75, a P-type AlGaN cladding layer 76a, an InGaN saturable absorbing layer 78, a p-type AlGaN cladding layer 76b, a p-type GaN contact layer 77, and a p-type electrode 79 are sequentially laminated. An n-type electrode 69 is provided below the n-type SIC substrate 70.
There is a possibility that the nitride semiconductor laser as disclosed in Japanese Laid-Open Patent Publication No. 9-191160 might not be preferably used for reproduction or recording/reproduction in an optical disk system, etc., since the range of output light wavelengths in which the nitride semiconductor laser can obtain self-pulsation is narrow. In such a nitride semiconductor laser including the saturable absorbing layer, satisfactory saturable absorption characteristics cannot be obtained unless carrier lifetime in the saturable absorbing layer including InGaN as a main component is short. In general, it is possible to shorten the carrier lifetime by doping the p-type InGaN saturable absorbing layer with Mg so as to enhance recombination of the carrier generated by light absorption. However, in practice, it is not easy to electrically activate almost an entire portion of the doped Mg provided in the InGaN saturable absorbing layer. Additionally, it is not easy to shorten apparent carrier lifetime since InGaN has a small carrier diffusion coefficient, whereby carriers generated in the saturable absorbing layer by light absorption are not easily diffused.
According to one aspect of the present invention, there is provided a semiconductor light-emitting device including a substrate; a light-emitting layer provided above the substrate; and a saturable absorbing layer provided above the substrate, the saturable absorbing layer having characteristics in which saturation of light absorption occurs, the semiconductor light-emitting device having self-pulsation characteristics due to the saturable absorbing layer and the semiconductor light-emitting device being characterized in that the saturable absorbing layer is doped with carbon.
In one embodiment of the invention, the saturable absorbing layer may be doped with a p-type dopant.
In one embodiment of the invention, the saturable absorbing layer may include a quantum well layer.
In one embodiment of the invention, the saturable absorbing layer may have a multiple quantum well structure including a plurality of quantum well layers and a plurality of barrier layers.
According to another aspect of the present invention, there is provided a method for producing a semiconductor light-emitting device, the method being characterized by comprising the steps of: forming a first nitride semiconductor layer on a first conductive nitride semiconductor substrate at a first growth temperature; sequentially forming above the first conductive nitride semiconductor layer a first conductive nitride semiconductor crack prevention layer at a second growth temperature differing from the first growth temperature, a first conductive nitride semiconductor cladding layer at the first growth temperature, and a first conductive nitride semiconductor guide layer at the first growth temperature; forming a first conductive nitride semiconductor active layer on the first conductive nitride semiconductor guide layer at a third growth temperature differing from the second growth temperature; sequentially forming above the first nitride semiconductor active layer a second conductive nitride semiconductor barrier layer and a second conductive nitride semiconductor guide layer at the first growth temperature; forming a saturable absorbing layer made of a nitride semiconductor on the second conductive nitride semiconductor guide layer at a fourth growth temperature differing from the third growth temperature; sequentially forming above the saturable absorbing layer made of a nitride semiconductor a second conductive nitride semiconductor cladding layer and a second conductive nitride semiconductor contact layer at the first growth temperature; and forming a ridge structure using a dry-etching treatment.
In one embodiment of the invention, the fourth growth temperature may be 700xc2x0 C. or less.
Thus, the invention described herein makes possible the advantage of providing: a semiconductor light-emitting device which can obtain stable self-pulsation characteristics by shortening the lifetime of a carrier generated by light absorption in a saturable absorbing layer in order to reduce the external optical feedback noise; and a method for producing such a device.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.